Light valve device with a protection circuit including a semiconductor device

Abstract: An aim is to provide a MOS integrated circuit with improved static electricity tolerance without sacrificing the high speed characteristic of an IC on a SOI substrate. A semiconductor device in which the thickness of a silicon single crystal thin film device forming layer (103) of an input/output protection circuit forming region (104) in a MOS integrated circuit on a SOI substrate (101,102) is made greater than the thickness of the silicon single crystal thin film device forming region (103) of a MOS integrated circuit forming region (105). The benefits can also be attained when the semiconductor device is used in a light valve device such as an active matrix liquid crystal display device.

Description: The present invention relates to a semiconductor device, and in particular to, but not exclusively to, an input/output protection circuit for a MOS type integrated circuit on a silicon single crystal thin film on an insulation layer, and also relates to a light valve device using the semiconductor device. Heretofore, an integrated circuit has been formed on a silicon, single crystal, thin film disposed on an insulation layer which is disposed on a substrate. An input/output protection circuit has been formed in this silicon, single crystal, thin film device forming layer. Figure 2 illustrates a conventional input/output protection circuit 104 for an integrated circuit formed on a silicon, single crystal, thin film 103 on an insulation layer 102. The input/output protection circuit 104 is formed in the silicon, single crystal, thin film device forming layer 103. This device forming layer 103 has the same thickness as a MOS type integrated circuit forming region 105. The input/output protection circuit 101 requires high speed operation and hitherto there has been a problem in that a diode or a field transistor used as a device for the input/output protection circuit cannot function sufficiently compared with being formed on a bulk silicon substrate. This is attributable to the fact that the thickness of the silicon, single crystal, thin film device forming layer has to be greatly reduced (less than 100 nm) so that the channel region of the MOS type transistor is completely depleted in order to provide the high speed characteristic of the MOS type integrated circuit on the silicon, single crystal, thin film on the insulation layer. If a normal diode or field transistor is formed on such a thin film, the junction portion of the P- and N-type diffusion regions 106, 107 is restricted. In the case where the diode is formed as shown in figure 4, the regions are restricted to the lateral sides thereof, thereby greatly reducing the junction area. In the case of a field transistor as shown in figure 6, a field oxide film 110 reaches as far as a buried insulation film 102, which results in the field transistor being unable to function. An application of such circuits is shown in figure 14, which illustrates an active matrix liquid crystal display device in which a silicon, single crystal, thin film 103 is disposed on an insulation layer 102. A driving substrate 501 is formed with a group of integrated circuits 502, 502' and a counter substrate 504 is formed with a counter electrode 503 disposed on a further silicon, single crystal, thin film 503. The driving substrate and the counter substrate are bonded at a gap with a liquid crystal 505 disposed in the gap. A light valve device has been known in which the silicon, single crystal, thin film device region includes field effect thin film transistors 506 arranged in a matrix for selectively supplying a voltage to the liquid crystal layer 505. Driving circuits 502, 502', including MOS type thin film transistors for driving the field effect thin film transistors, each have an input terminal portion 507 for inputting signals from the outside to the driving circuit. The light valve device can incorporate a driving circuit formed by single crystal, silicon LSI technology and has features of being able to reduce the number of connection terminals to the outside, having high precision and a microscopic size. However, in the manufacturing step for the liquid crystal display device, since, for example, the semiconductor device is subjected to high speed rotation or cloth rubbing for the orientating treatment of liquid crystals, there has been a problem in that electrostatic charge is generated leading to frequent inferiorities. Also electrostatic charge which is in the atmosphere during manufacture and where the finished product is placed in this atmosphere, the charge intrudes into the input terminal portion 507 where a conductive film is exposed and destroys the device. EP 474474A describes a semiconductor light valve device and process for fabricating the same. EP 480373A describes a thin film semiconductor device formed over an insulating film. It is an object of the present invention to overcome the foregoing problems and provide a MOS integrated circuit formed on a silicon single crystal thin film on an insulation layer having an input/output protection circuit capable of functioning sufficiently and a light valve device using the integrated circuit and provided with sufficient countermeasure for electrostatic charges. According to a first aspect of the present invention, there is provided a semiconductor device as defined in claim 1. According to a second aspect of the present invention, there is provided a light valve device as defined in claim 7. The means adopted in the semiconductor device and the light valve device according to the present invention for attaining the aforegoing object, in order that a diode or a field transistor used as a device for an input/output protection circuit can provide sufficient function in a MOS integrated circuit on a silicon single crystal thin film on an insulation layer, comprises the steps of: (1) making the thickness of a silicon single crystal thin film device forming layer in an input/output protection circuit forming region greater than the thickness of a silicon single crystal thin film device forming region in a MOS integrated circuit forming region requiring high speed operation; (2) forming an input/output protection circuit on a silicon single crystal substrate with a silicon single crystal thin film device forming layer and a buried insulated film on the silicon single crystal thin film on the insulation layer removed therefrom; and (3) forming a diode used as an element for the input/output protection device with a silicon single crystal thin film device forming layer on the single crystal thin film on the insulation layer and a polycrystal silicon layer formed on the silicon single crystal thin film device forming layer. Thus, the MOS integrated circuit requiring high speed operation is formed in the single crystal, thin film, device forming layer with a film thickness being set such that a channel region is completely depleted (ie. less than about 100 nm) upon operation of a MOS transistor by way of the buried insulation film on the silicon single crystal substrate, so that electrostatic tolerance can be improved without sacrificing the high speed characteristic of the IC on the silicon, single crystal, thin film on the insulation layer. Further, a light valve device protected from electrostatic influence, such as during the manufacturing stage, can be manufactured by using a semiconductor device having the integrated circuit formed thereon. In the semiconductor device according to the present invention, because the film thickness of the silicon, single crystal, thin film device forming layer of the input/output protection circuit forming region is made greater so that the input/output protection device can provide sufficient function in the MOS integrated circuit on the silicon, single crystal, thin film on the insulation layer, the input/output protection circuit is formed on a silicon, single crystal substrate with the silicon, single crystal, thin film device forming layer on an SOI substrate and the buried insulation film removed. The diode used as the element for the input/output protection circuit is formed with the silicon, single crystal, thin film, device forming layer on the SOI substrate and the polycrystal silicon layer formed on the silicon, single crystal, thin film, device forming layer. Thus the diode or field transistor used as the element for the input/output protection circuit can provide a function equal to that in the case where it is formed on a bulk silicon substrate, and a MOS integrated circuit having sufficient resistance against the application of static electricity can be formed. Embodiments of the present invention will now be described with reference to the accompanying drawings, of which: Figure 1 is a schematic cross sectional view illustrating an embodiment of a semiconductor device according to the present invention; Figure 2 is a schematic cross sectional view illustrating a conventional semiconductor device; Figure 3 is a schematic cross sectional view illustrating an embodiment of a semiconductor device according to the present invention; Figure 4 is a schematic cross sectional view of a diode used an element for an input/output protection circuit of a conventional semiconductor device; Figure 5 is a schematic cross sectional view of a diode used as an element for an input/output protection circuit of a semiconductor device according to the present invention; Figure 6 is a schematic cross sectional view of a field transistor used as an element for an input/output protection circuit of a conventional semiconductor device; Figure 7 is a schematic cross sectional view of a field transistor used as an element for an input/output protection circuit of a semiconductor device according to the present invention; Figure 8 is a schematic cross sectional view illustrating an embodiment of a semiconductor device according to the present invention; Figure 9 is a schematic cross sectional view of a diode used as an element for an input/output protection circuit of a conventional semiconductor device; Figure 10 is a schematic cross sectional view of a diode used as an element for an input/output protection circuit of a semiconductor device according to the present invention; Figure 11 is a schematic cross sectional view of a field transistor used as an element for an input/output protection circuit of a conventional semiconductor device; Figure 12 is a schematic cross sectional view of a field transistor used as an element for an input/output protection circuit of a semiconductor device according to the present invention; Figure 13 is a schematic cross sectional view illustrating an embodiment of a diode used as an element for an input/output protection circuit of a semiconductor device according to the present invention; Figure 14 is a schematic perspective view of a conventional light valve device; and Figure 15 is a schematic perspective view illustrating an embodiment of a light valve device according to the present invention. A description of preferred embodiments of the present invention follows, with reference to the drawings. Figure 1 is a schematic cross sectional view illustrating one embodiment of a semiconductor device according to the present invention. A silicon, single crystal, thin film, device forming layer 103 is formed by way of a buried insulation film 102 on a silicon, single crystal, substrate 101. In this embodiment, the thickness of the silicon single crystal thin film device forming layer 103 in a MOS integrated circuit forming region 105 requiring high speed operation is determined such that a channel region is completely depleted upon operation of the MOS transistor. For example, less than about 100 nm. On the other hand, the thickness of the silicon, single crystal, thin film, device forming layer 103 in an input/output protection circuit forming region 104 is made greater than the thickness of the MOS integrated circuit forming region 105. The thickness of the silicon single crystal thin film device forming layer 103 in the input/output protection circuit forming region 104 is made greater than the thickness of a first conduction type diffusion region of a diode in the case where the diode is used as an element for the input/output protection circuit. Alternatively, it has such a thickness that the bottom of a field oxide film of a field transistor does not reach the buried insulation film of the SOI substrate in the case where a field transistor is used as an element for the input/output protection circuit. Fig. 3 is a schematic cross sectional view illustrating another embodiment of the semiconductor device according to the present invention. This embodiment is different from that shown in figure 1 in that a MOS integrated circuit forming region 201, which does not require high speed operation, is also formed in addition to the MOS integrated circuit forming region 105 requiring high speed operation. The thickness of the silicon single crystal thin film device forming layer 103 in the MOS integrated circuit forming region 201 is the same as the thickness of the silicon single crystal thin film device forming layer 103 in the input/output protection circuit forming region 104. The thickness of the silicon single crystal thin film device forming layer 103 of the MOS integrated circuit forming region 105 is determined such that a channel region is completely depleted upon operation of the MOS transistor. For example less than about 100 nm. Figure 5 is a schematic cross sectional view illustrating an embodiment of a diode used an element for an input/output protection circuit of a semiconductor device according to the present invention. Although in a conventional diode on a silicon single crystal thin film device forming layer 103 shown in figure 4, a junction portion for the P- and N-type diffusion regions is restricted to the lateral sides thereof which remarkably reduces the junction area, in the diode used as a device for the input/output protection device of the semiconductor device according to the present invention shown in figure 5, since the bottom of an N-type diffusion layer 106 is also in contact with a P-type diffusion layer 107, the junction area is increased. In addition the concentration of the electric field can be moderated, and sufficient protection can be realised. Figure 7 is a schematic cross sectional view illustrating an embodiment of a field transistor used as an element for an input/output protection circuit of a semiconductor device according to the present invention. The field transistor comprises a source 108, a drain 109, a field oxide film 110 acting as a gate insulation film and a gate electrode 111. In the field transistor on the conventional silicon single crystal thin film device forming layer 103 shown in Figure 6, the field oxide film 110 reaches as far as the insulation film 102 and so fails to form a field transistor. On the other hand, in the field transistor used as the element for the input/output protection device of the semiconductor device according to the present invention, since it is formed in the silicon single crystal thin film device forming layer 103 having such a thickness that the bottom of the field oxide film of the field transistor does not reach the buried insulation film 102 of the SOI substrate, a satisfactory structure can be obtained. In the embodiments shown in Figs. 1, 3, 5 and 7, since the thickness of the silicon single crystal thin film device forming layer of the input/output protection circuit forming region is increased such that the input/output protection device can sufficiently realise its function, the diode or the field transistor used as the device for the input/output protection circuit can provide a function equal to that in the case where it is formed on a bulk silicon substrate. Thus a MOS integrated circuit having sufficient tolerance to the application of static electricity can be formed. Figure 8 is a schematic cross sectional view showing another embodiment of a semiconductor according to the present invention. A silicon single crystal thin film device forming layer 103 is formed by way of a buried insulation film 102 on a silicon single crystal substrate 101. In this embodiment, the silicon single crystal thin film device forming layer 103 and the buried insulation layer 102 of an input/output protection circuit forming region 104 are removed, and the input/output protection circuit is formed on the silicon single crystal substrate 101. On the other hand, the silicon single crystal thin film device forming layer 103 of a MOS integrated circuit forming region 105, requiring high speed operation, is formed by way of a buried insulation layer 102 on the silicon single crystal substrate 101. The thickness of the silicon single crystal thin film device forming layer 103 is determined such that a channel region is completely depleted upon operation of a MOS transistor. For example, less than about 100 nm. Figure 10 is a schematic cross sectional view illustrating an embodiment of a diode used as an element for an input/output protection circuit of a semiconductor device according to the present invention. In a diode shown in figure 9 on a conventional silicon single crystal thin film device forming layer 103, a junction portion for the P- and N-type diffusion regions is restricted to the lateral sides thereof which remarkably reduces the junction area. On the other hand, in a diode used as an element for an input/output protection circuit of a semiconductor device according to the present invention, since the bottom of an N-type diffusion layer 106 is also in contact with the P-type diffusion region 107, the junction area is increased. Moreover, the concentration of the electric field can be moderated and the protection circuit can sufficiently realise its function. Figure 12 is a schematic cross sectional view illustrating an embodiment of a field transistor used as an element for an input/output protection circuit or a semiconductor device according to the present invention. In the field transistor on a conventional silicon single crystal thin film device forming layer 103 shown in figure 11, a field oxide film reaches as far as a buried insulation film: thus failing to form the field transistor. On the other hand, since the field transistor used as the device for the input/output protection circuit of the semiconductor device according to the present invention shown in figure 6 is formed on the silicon single crystal substrate 101, a satisfactory structure can be obtained. According to the embodiments shown in figures 8, 10 and 12, since the input/output protection circuit is formed on the single silicon crystal substrate with the silicon single crystal thin film device forming layer and the buried insulation film on the SOI substrate removed, the diode or field transistor used as the element for the input/output protection circuit can provide a function equal to that in the case where it is formed on a bulk silicon substrate. Thus a MOS integrated circuit having sufficient tolerance to the application of static electricity can be formed. Figure 13 is a schematic cross sectional view illustrating an embodiment of a diode used as an element for an input/output protection circuit according to the present invention. A silicon single crystal thin film device forming layer 103 is formed by way of a buried insulation film 102 on a silicon single crystal substrate 101. In this embodiment, the diode used as the element for the input/output protection circuit is formed with a silicon single crystal thin film device forming layer 103 on an SOI substrate and an N-type polycrystal silicon layer 301 formed on the silicon single crystal thin film device forming layer 103. In the diode in the conventional silicon single crystal thin film device forming layer 103 shown in figure 4, a junction portion for the P-type diffusion region 107 and the N-type diffusion region 106 is restricted to the lateral sides thereof and so greatly reduces the junction area. On the other hand, in the diode used as the element for the input/output protection circuit of the semiconductor device according to the present invention, since the N-type region is formed with the N-type polycrystal silicon layer 301, and the bottom of the N-type polycrystal silicon layer 301 is in contact with the P-type diffusion layer 107, the junction area is increased. Moreover the concentration of the electric field can be moderated and the protection circuit can sufficiently realise its function. Although a description of Figure 13 is set forth above for a case where the N-type polycrystal silicon layer is formed on the silicon single crystal thin film device forming layer 103 and the P-type diffusion region 107 is formed in the silicon single crystal thin film device forming layer, a P-type polycrystal silicon layer may be formed in the silicon single crystal thin film device forming layer 103, while an N-type diffusion layer may be formed in the silicon single crystal thin film device forming layer 103. According to the embodiment shown in Figure 13, the diode used as the element for the input/output protection circuit in a MOS integrated circuit on a silicon single crystal thin film on an insulation layer can provide a function equal to that in the case where it is formed within a bulk silicon substrate, and a MOS integrated circuit having sufficient resistance to the application of static electricity can be formed. Figure 15 is a schematic perspective view illustrating an embodiment of a light valve device according to the present invention. A silicon single crystal thin film 103 is disposed on an insulation layer 102. A driving substrate 501, formed with a group of integrated circuits 502, 502', and a counter substrate 504 formed with a counter electrode 503 are disposed on the silicon single crystal thin film 103. The driving substrate 501 and the counter substrate 504 being bonded at a gap and a liquid crystal layer 505 disposed in the gap. The group of integrated circuits 502, 502' includes an input protection circuit region or output protection circuit region 507. The silicon single crystal thin film device region 103 includes field effect thin film transistors 506 arranged in a matrix for supplying voltage selectively to the liquid crystal layer 505, driving circuits 502, 502' including the MOS thin film transistors for driving the field effect thin film transistors, and the input protection circuit region or the output protection circuit region 507 is constituted by the group of input/output protection devices 508 according to the present invention shown in Figures 1, 3, 5, 7, 8, 10, 12 and 13. According to the embodiment shown in Figure 15, the semiconductor device used for the light valve device can be provided with effective protection against static electricity either during manufacturing or as a completed product. As has been described above, one can fully attain the function of the input/output protection circuit in the MOS integrated circuit on the silicon single crystal thin film on the insulation layer according to the present invention, since (1) the thickness of the silicon single crystal thin film device forming layer of the input/output protection circuit forming region is increased, or (2) the input/output protection circuit is formed on the silicon single crystal substrate with the silicon crystal thin film device forming layer and the buried insulation layer on the SOI substrate removed, or (3) the diode used as the element for the input/output protection circuit is formed with the silicon single crystal single device forming layer on the SOI substrate and the polycrystal silicon layer formed on the silicon single crystal device forming layer. Thus the diode or the field transistor used as the element for the input/output protection circuit can provide a function equal to that in the case of being formed on a bulk silicon substrate. Moreover, a MOS integrated circuit having sufficient tolerance to the application of static electricity can be formed. On the other hand, the MOS integrated circuit requiring high speed operation is formed on a silicon single crystal thin film device forming layer formed by way of a buried insulation film on the silicon single crystal substrate and having a film thickness determined such that the channel region is completely depleted upon operation of the MOS transistor. For example, less than about 100nm. Thus static electricity tolerance can be improved without sacrificing the high speed characteristic of the IC on the SOI substrate. Further, a highly precise and compact light valve device having a driving circuit comprising silicon single crystals and sufficiently protected against electrostatic charge can be attained. The aforegoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention, as defined in the appended claims.

Claims: A semiconductor device having a substrate (101) on which is disposed successively an insulating layer (102) and a device forming layer (103) comprising a silicon single crystal thin film, said device having an integrated circuit region (105) in which a MOS integrated circuit including a MOS transistor is formed in said device forming layer (103) integrated circuit characterised in that said device has a protection circuit region (104) in which an input/output protection circuit including a diode or a transistor is formed adjacent said region, the protection circuit being realised so as to protect said integrated circuit region from electrostatic charges, and being formed separated from said insulating layer (102) whereby to permit said diode or transistor to be formed in such a way as to moderate electrical stress concentrations and the thickness of said device forming layer (103) in said integrated circuit region is such that a channel region is substantially completely depleted upon operation of the MOS transistor. A semiconductor device as claimed in claim 1, wherein the protection circuit is also formed in said device forming layer (103) and that the thickness of the device forming layer (103) of the protection circuit region is greater than the thickness of the device forming layer of the integrated region (105). A semiconductor device as claimed in claim 2, wherein the protection circuit includes a diode with a diffusion region (106) having a first conduction type formed in the device forming layer (107) having a second conduction type in the protection circuit region, and the diffusion region of said first conduction type has a thickness such that it does not contact the insulting layer. A semiconductor device as claimed in claim 2, wherein the protection circuit includes a field transistor comprising a source region (108) and a drain region (109) which are formed in the device forming layer, a field oxide film (110) formed on the device forming layer and a gate electrode (111) formed on the field oxide film, the bottom of the field oxide film of said transistor being separated from the insulating layer. A semiconductor device as claimed in claim 1, wherein protection circuit is formed directly in the substrate. A semiconductor device as claimed in claim 1, wherein the protection circuit includes a diode comprising a polycrystal silicon layer (301) formed on the device forming layer. A light valve device comprising a driving substrate (101) on which is disposed successively an insulating layer (102) and a silicon single crystal thin film (103), the driving substrate being formed with a group of integrated circuits (502, 502', 506) and a counter substrate (504) formed with a counter electrode (503), said driving substrate and said counter substrate are bonded at a gap, and a liquid crystal layer (505) is disposed in said gap, wherein the group of said integrated circuits includes an input or output protection circuit region (507) and a silicon single crystal thin film device region, said input or output protection circuit region being formed separated from said insulating layer and adjacent said silicon single crystal thin film device region, said silicon single crystal thin film device region includes field effect thin film transistors arranged in a matrix for selectively supplying voltage to said liquid crystal layer and a driving circuit including MOS thin film transistors for driving said field effect thin film transistors, and said input or output protection circuit region includes a protection device electrically connected to said MOS thin film transistors, said protection device being realised so as to protect said MOS thin film transistors against destruction by static electricity, and a thickness of said silicon single crystal thin film device region is such that a channel region is substantially completely depleted upon operation of the MOS transistors.

Inventors: TAKAHASHI KUNIHIRO, TAKASU HIROAKI, YAMAZAKI TSUNEO, TAKAHASHI, KUNIHIRO, TAKASU, HIROAKI, YAMAZAKI, TSUNEO, TAKAHASHI, KUNIHIRO, C/O SEIKO INSTRUMENTS INC., TAKASU, HIROAKI, C/O SEIKO INSTRUMENTS INC., YAMAZAKI, TSUNEO, C/O SEIKO INSTRUMENTS INC.
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1994
Main classification: G02F1
IPC codes: H01L27, H01L29, G02F1, H01L21